Increasing storage capacity is a goal in the data storage industry. Data storage products, such as magnetic disk drives, optical disk drives and magnetic tape drives, store tracks of digital information on a moving medium using one or more read/write heads. As track widths decrease, the recording head must be more accurately positioned to compensate for mechanical disturbance that degrades the head signal by mis-positioning the head relative to the track. Because micro-actuators operating with increased bandwidth remove more mechanical disturbance, a method and apparatus is desired that improves the mechanical bandwidth of the micro-actuator and the electrical bandwidth of the position control signal.
The data storage industry has a large investment in manufacturing and test equipment. This equipment also requires a fine positioning mechanism with improved bandwidth.
Manufacturing and test equipment for the magnetic and optical disk drive industries comprise a head tester, a disk tester, a disk certifier, a media certifier, a servo track writer, a head/disk tester and a spinstand. Head/disk testers are used to qualify the head and disks and to extend recording technology. Media certifiers characterize the magnetic and mechanical properties of a disk and can improve performance by burnishing the disk surface to reduce head/disk spacing. Servo track writers record servo patterns on disks; the amount of written-in run-out and the placement accuracy of the servo pattern affect track squeeze, track density and how well the disk drive performs. A spinstand is an equipment subsystem that mechanically positions a head so it can fly on a spinning disk at the desired radius, skew angle and z-height using one or more positioning mechanisms. Added to the spinstand subsystem are sensors, mechanics, electronics and software to measure various aspects of disk drive performance. All of these types of manufacturing and test equipment need a fine positioning mechanism with improved bandwidth.
By extension, the same fine positioning improvements needed by the magnetic and optical disk drive industries are needed by the tape drive industry. Whereas magnetic and optical disk equipment use a spinstand where the disk medium is rotated, tape drive equipment use a test stand wherein the tape medium is moved linearly in either a loop or reel to reel. Tape heads need to be precisely located on or about the data track on the moving medium. Mechanical disturbance occurs in this equipment that displace the head relative to the track. This equipment too needs a fine positioning mechanism with improved bandwidth as track widths decrease.
Data storage manufacturing and test equipment for magnetic, optical and tape drives all have a head, a medium, a motor that moves the medium, tracks of data recorded on a medium and a requirement for fine positioning of a head on or about a track.
Regarding the notion of positioning a head “on or about a track,” the “on track” refers to positioning the head at the track center as best as possible and the “about a track” refers to purposely positioning the head a defined distance off-track from track center as is required for measuring track profile, evaluating error rates and writing servo patterns. All references to positioning are along the off-track axis, and more specifically, the axis perpendicular to the recorded track and in-plane with the disk surface, except where noted. In this disclosure, “head” is also synonymous with “head assembly” and “HGA.” In the disk drive industry, an “HGA” is a “head gimbal assembly” comprising a suspension assembly, slider and transducer. Also, the word “micro-actuator” is used as one type of “fine positioning mechanism.”
For many years the head was positioned on or about the track by simply positioning the head at the desired location with no active positioning. Mechanical disturbance, such as due to floor vibration, thermal drift and vibration internal to the equipment, caused minimal degradation to the head signal. However as track widths have decreased over time, mechanical disturbance has become more significant, causing the head to move on and off track and the head signal to degrade.
Manufacturing and test equipment now use track following to generate a position control signal that drives a micro-actuator to actively position the head on or about a track. Micro-actuators are typically made from voice coil or piezoelectric motors. These micro-actuators have difficulty moving the mass of the head and the intervening hardware quickly and accurately enough and thus suffer from insufficient mechanical bandwidth.
The most stringent demand for a high bandwidth micro-actuator and position control signal comes from the disk drive industry. While the foregoing disclosure uses a spinstand for the disk drive industry as its example, it is understood that the present invention is not limited to a spinstand.
The mechanical bandwidth of a micro-actuator can be improved by reducing moving mass, increasing mechanical stiffness, reducing vibration and maximizing single axis, in-plane motion by minimizing off-axis distortion.
Related art U.S. Pat. No. 6,006,614 to Guzik et al. entitled “Apparatus and method for improving dynamic characteristics of a fine positioning mechanism in a magnetic head/disk tester” and U.S. Patent Application 20020057517 to Takagi et al. entitled “Head clamping apparatus for magnetic disk tester and magnetic disk tester” describe micro-actuators that fine position a head on or about a track. Both use a piezo actuator that “expands and contracts” deforming a second structure whose mechanical purpose is to convert the deforming motion into a single axis, in-plane displacement. In U.S. Pat. No. 6,006,614, the deformed structure is a “hollow parallelepiped.” In U.S. Patent Application 20020057517 the deformed structure is a “parallel plate spring” structure. In both related arts, the intervening hardware between the piezo actuator and head has spring, frame and cantilever structures.
In the context of improved bandwidth, the above related art's fine positioning mechanism displaces excessive mass between the piezo actuator and the head, has insufficient stiffness, has undesirable vibration modes and has off-axis distortion. Regarding mass, the deformable structure adds mass. Regarding stiffness, vibration and off-axis distortion, the deformable structure is purposely made less stiff on the axis of motion which inadvertently decreases the stiffness on other axes, causing the structure to be more prone to vibration and off-axis distortion on the pitch, roll or yaw axes. Higher mass and lower stiffness reduce resonance frequencies. Resonance modes within operating bandwidth of the micro-actuator are unacceptable. Furthermore, frame, spring and cantilever structures have less stiffness and vibrate more than solid, fully supported structures of equal mass.
Micro-actuators need to provide single axis, in-plane motion across the entire operating bandwidth. What is desired is an apparatus and a method for a fine positioning mechanism that avoids frame, spring and cantilever structures and reduces moving mass, increases mechanical stiffness, reduces vibration and reduces off-axis distortion.
A micro-actuator with improved mechanical bandwidth cannot be fully utilized if the position control signal driving the micro-actuator does not have a matching, improved electrical bandwidth.
In related art, the position control signal is the output of a position control system whose feedback signal comes from a position sensor and/or from track following. For example, related art U.S. Patent Application 20020057517 also describes a positioning control system that switches between a precise positioning mode that uses an optical sensor to sense the position of the micro-actuator and a track following mode that senses the relative position of the head to the track.
Track-following is an embedded servo technique that is used throughout the disk drive industry for actively positioning a head relative to a track. In track following, servo bursts are recorded on a disk, a head reads the servo bursts to sense the head's position relative to the track, a position error signal (PES) is created and a control loop generates a compensating track following signal that drives a micro-actuator and positions the head on the track. Track following is effective because it measures position where it counts: exactly between the head and track where all sources of mechanical disturbance are sensed at once.
However, track following provides a position feedback signal that has limited bandwidth because head position is sampled. There is no positioning information between servo bursts. The bandwidth of the head position signal is determined from the number of servo bursts per disk revolution and the disk spin rate (RPM) or equivalently, the sampling rate. Increasing the number of servo bursts per disk revolution increases the bandwidth. The number of servo bursts is limited in a practical sense because any disk area set aside for recording servo bursts takes away disk area for storing data. For example, it is common for 10 percent of the disk area to be dedicated to the embedded servo leaving 90 percent of the disk area for recording data, for a disk efficiency of 90%. Track following bandwidth has a practical limit determined by disk efficiency. Track following bandwidth is further limited because the position signal from track following must be over-sampled. An industry guideline advocates a minimum of 10 times over-sampling. Because phase delay and aliasing result from insufficient over-sampling and filtering, an even greater over-sampling factor is desired for manufacturing and test equipment.
Increasing the electrical bandwidth of the position control signal requires that the bandwidth of the positioning control system be increased. The bandwidth of a positioning control system is dependent upon the mechanical bandwidth of the micro-actuator, the electrical bandwidth of the position sensing signal and the open loop gain cross-over frequency. Increasing the bandwidth of the positioning control system faces several challenges in addition to the limited electrical bandwidth of the track following signal and the mechanical bandwidth of the micro-actuator. Mechanical resonance in the head suspension assembly and other structures cause Bode plot peaks that challenge loop stability. The micro-actuator and position sensor are not collocated in track following mode. Finally, the head's suspension assembly is not adequately stiff on the off-track axis causing head actuation loss.
Positioning control system bandwidth has historically been greater disk drives than manufacturing and test equipment, even when both use track following, because disk drives have a higher mechanical bandwidth head positioning mechanism and because their entire electromechanical system is highly optimized. For example, disk drives can optimize performance by using notch filters whose center frequencies match specific resonance frequencies of the head and other structural components. In contrast, manufacturing and test equipment is used to test a variety of heads and disks, which inhibits a high level of optimization. Manufacturing and test equipment has the same bandwidth limitations as disk drives plus the added limitations of being required to quickly adapt to many different types of heads and disks.
The purpose of a micro-actuator and its position control signal is to remove mechanical disturbance that mis-positions the head up to the bandwidth of the positioning control system and hence the importance of improving bandwidth. Sources of mechanical disturbance in a spinstand comprise spindle run-out, disk flutter, head vibration, spinstand vibration, windage and thermal drift. Mechanical disturbance from the rotating spindle and disk are decomposed into repeatable run-out (RRO) and non-repeatable run-out (NRRO) components. When servo track writers record servo patterns, mechanical disturbance perturbs what would otherwise be a perfectly circular track, with the resulting distortion called written-in run-out (WI-RO). What is needed is an apparatus and a method for improving bandwidth of a fine positioning mechanism in data storage manufacturing and test equipment that separately senses and compensates different types of mechanical disturbance.